Field of the Invention
The present invention relates to an electronic control device equipped with a micro-processor and a multi-channel A-to-D converter both fed from a constant-voltage power supply, and more particularly, to an electronic control device improved so as to constantly monitor the presence or absence of a power-supply abnormality in a constant-voltage power supply.
Description of the Related Art
As in-vehicle electronic control devices for automobile, there is a voltage monitor device equipped with a micro-processor that monitors a variation of a digital conversion value by monitoring a power-supply voltage to be fed to a reference power-supply terminal of an A-to-D converter.
For example, JP-A-09-027749 (Patent Document 1) (see FIG. 1) discloses a voltage monitor device including a circuit (resistor 1 and zener diode 2) that generates a constant voltage Vz from an analog input voltage (for example, Vcc) to be monitored in such a manner that the constant voltage Vz has a lower value than the analog input voltage, an A-to-D converter 3, and further a CPU 4 that monitors a variation of a digital value outputted from the A-to-D converter 3 connected inversely to a normal use of an A-to-D converter in such a manner that the analog input voltage to be monitored is given to a reference voltage terminal 3s and the constant voltage Vz is given to an analog input terminal 3a. According to this voltage monitor device, it becomes possible to provide a device that not only makes it possible to monitor a voltage variation accurately, but also makes it easy to change a comparative voltage.
Also, JP-A-2009-022152 (Patent Document 2) (see FIG. 14) discloses a constant-voltage power supply for in-vehicle electronic control device having multiple types of stabilized output voltages and configured to perform comprehensive abnormality processing by detecting the presence or absence of an abnormality in each output voltage. In order to perform comprehensive abnormality processing by detecting the presence or absence of an abnormality in each output voltage, the constant-voltage power supply generates an output voltage Vad of 5 V as a high-accurate low-capacity power supply, an output voltage Vif of 5 V as a low-accurate high-capacity power supply, and an output voltage Vcp of 3.3 V as a low-accurate high-capacity power supply. Further, the constant-voltage power supply generates at least one of an output voltage Vup of 2.8 V as a low-accurate low-capacity power supply and an output voltage Vsb of 3.3 V as a high-accurate low-capacity power supply. A determination signal input circuit compares a divided voltage of the output voltage Vif, a divided voltage of the output voltage Vcp, a divided voltage of the output voltage Vup, and a divided voltage of the output voltage Vsb in reference, for example, to a divided voltage of the output voltage Vad. The determination signal input circuit then processes the comparison results by logic synthesis and inputs relative voltage information ER2, ER3, ER4, and ER5 into the micro-processor. The micro-processor makes a comprehensive determination including the comparative reference voltage on the basis of the relative voltage information and notifies an abnormality or saves abnormality occurrence information. The reference numerals and the signs used above are those used in respective Patent Documents.
[Patent Document 1] JP-A-09-027749 (Abstract and FIG. 1)
[Patent Document 2] JP-A-2009-022152 (Abstract and FIG. 14)
According to the voltage monitor device disclosed in Patent Document 1, by configuring in such a manner that the analog input voltage to be monitored is given to the reference voltage terminal and the constant voltage (reference voltage) is given to the analog input terminal, the constant voltage used as the reference voltage can take a lower value than the analog input voltage to be monitored. Hence, the constant voltage can be generated easily from the analog input voltage (for example, power-supply voltage Vcc) to be monitored using a simple circuit, such as a zener diode.
However, voltage characteristics vary from one zener diode to another and the reference voltage to be monitored varies in a similar manner. Hence, the obtained result is too uncertain to make a determination as to which one is correct. In order to make the obtained result certain, temperature characteristics have to be corrected beforehand so as to accommodate a change of the environmental temperature in a broad range by performing an initial calibration corresponding to characteristics of an actual zener diode. Hence, there is a problem that the constant voltage (reference voltage) is by no means inexpensive.
According to the in-vehicle electronic control device disclosed in Patent Document 2, an output voltage of the constant-voltage power-supply circuit having a high degree of output voltage accuracy among multiple constant-voltage power-supply circuits is used as the comparative reference voltage. The presence or absence of an individual abnormality is detected by a band comparison as to whether output voltages of multiple constant-voltage power-supply circuits are within an allowable variation range. Hence, when the high-accurate voltage used as the comparative reference has a pulsation variation, a relative comparison becomes difficult. In such a case, there arises a problem that not only an accurate abnormality determination cannot be made, but also magnitude of the pulsation variation cannot be understood quantitatively.